1. Field of the Invention
This invention relates to methods and apparatus for enhancing neurophysiologic performance, such as sensorimotor control and neuroplasticity, by combining improved function of sensory cells with pre-defined physical activity and use of certain devices.
2. Description of Related Art
The nervous system of mammals is a complex set of interrelated and interacting sub-systems. The sub-systems are categorized and named both by their anatomic positions and by their function. At the highest level, the nervous system is divided into central and peripheral nervous systems. The central nervous system (CNS) is comprised of the brain and spinal cord; the peripheral nervous system (PNS) subsumes all the remaining neural structures found outside the CNS. The PNS is further divided functionally into the somatic (voluntary) and autonomic (involuntary) nervous systems. The PNS can also be described structurally as being comprised of afferent (sensory) nerves, which carry information toward the CNS, and efferent (motor) nerves, which carry commands away from the CNS.
Interconnections between afferent and efferent nerves are found in the spinal cord and brain. Taken together, certain groupings of afferent and efferent nerves constitute sensorimotor “loops” that are required to achieve coordinated movements in the face of perturbations from the environment and changes in volitional intent. In the periphery (trunk, upper extremities, and lower extremities), afferent nerves carry sensory information arising from special neurons that are sensitive to pain, temperature, and mechanical stimuli such as touch and vibration at the skin surface, and position, force, and stretch of deeper structures such as muscles, tendons, ligaments, and joint capsule. The term “proprioception” generally applies to sensory information directly relevant to limb position sense and muscle contraction. Combined with tactile (touch) sensation, mechanical sensory information is collectively known as “somatosensation.”
Specialized “mechanoreceptor” neurons transduce mechanical stimuli from the body's interaction with the environment into electrical signals that can be transmitted and interpreted by the nervous system. Pacinian corpuscles in the skin fire in response to touch pressure. Muscle spindles, found interspersed in skeletal muscle tissue, report on the state of stretch of the surrounding muscle. Golgi tendon organs sense the level of force in the tendon. Free nerve endings in structures surrounding joints (ligaments, meniscus, etc.) provide additional information about joint position. Some of these mechanoreceptor systems are thought to interact directly via excitatory and inhibitory synapses and descending pathways to modulate the performance or interpretation of signals from other mechanoreceptor systems.
Sensory cells of all types are typically threshold-based units. That is, if the stimulus to a sensory cell is of insufficient magnitude, the cell will not activate and begin signaling. Such a stimulus is called “subthreshold.” A stimulus that is above the threshold is called “suprathreshold.”
Connections within the nervous system—brain, spinal cord, and peripheral nerves—are highly changeable in the face of demands placed on the body: new forms of activity, pathologies, and injuries. In healthy individuals, these neurological changes allow for the acquisition of new physical skills, a process termed “motor learning.” Following certain types of soft tissue injury (e.g. rupture of the anterior cruciate ligament of the knee, a structure known to be rich in mechanoreceptors), and subsequent medical efforts such as surgery used to repair the damage, the nervous system can undergo compensatory changes to accommodate for loss of the natural sensory neurons. Similar PNS and CNS nervous system changes account for some individual's ability to regain lost motor function following spinal or brain injuries. Taken together, these structural changes in the nervous systems are termed “neuroplasticity” or “neuroplastic changes.”
Recent research has established that afferent (sensory) activity from the periphery is one of the key drivers of neuroplastic changes in the nervous system, both in the PNS and CNS.
The present invention focuses on mechanical sensory neurons in the periphery and the role they play, specifically, in sensorimotor control and in inducing neuroplastic changes in the nervous system. In this invention, we combine prior art methods of improving the performance of individual sensory cells with novel methods and apparatus to achieve improvements in sensorimotor control and neuroplasticity. Importantly, the nature of the improved sensory cell performance is that the natural firing rate in response to environmental stimuli is increased in an information-rich fashion. That is, the increased sensory cell firing is concordant with limb function and hence is not gratuitous or uncoordinated in nature.
Electrical stimulation of tissue has been used for various therapeutic purposes including stimulating muscle activity, relieving pain, and producing sensation. The sequence of effects produced by electrical stimulation, as its intensity is increased, generally follows a pattern of a perception of an electrical sensation (such as tingling), an increase in sensation, fasciculation muscle contraction, pain, and then injury in the form of electrical burns or cardiac arrhythmias.
In the past, pulsed electrical waveforms having an adjustable pulse duration, intensity and pulse width have been applied to a particular area of the human body for therapeutic purposes to suppress pain. Electrical waveform therapy, such as that disclosed in U.S. Pat. No. 5,487,759 to Bastyr, et al. has been used for symptomatic relief and management of chronic, post surgical and posttraumatic acute pain and for inducing muscle contraction for the retardation of atrophy.
Stimulation below perception levels (i.e. subthreshold stimulation) used to enhance the function of sensory cells is described in U.S. Pat. Nos. 5,782,873 and 6,032,074 to Collins, the entire contents of which are incorporated by reference. Collins discloses a method and apparatus for improving the function of sensory cells by effectively lowering their threshold of firing. Briefly, a subthreshold stimulation, or “bias signal,” is input to the sensory neuron that predisposes the neuron to firing, without actually causing it to fire. In one preferred embodiment, the bias signal is a broadband signal containing many frequencies, often termed “noise.” Since sensory cells are typically threshold-based units, lowering the sensory cell threshold decreases the level of outside stimulus needed to cause the sensory cell to respond (i.e. fire). Thus, the sensory cell, in the presence of the bias signal, is expected to respond to stimulus intensities that would normally be considered subthreshold to the neuron in the absence of noise. Both electrical and mechanical modalities of bias signal, used individually or in combination, may be used to effect the lowering of sensory neuron detection threshold.